Matching optical metrology tools using spectra enhancement

ABSTRACT

Optical metrology tools are matched by obtaining a first set of measured diffraction signals, which was measured using a first optical metrology tool, and a second set of measured diffraction signals, which was measured using a second optical metrology tool. A first spectra-shift offset is generated based on the difference between the first set of measured diffraction signals and the second set of measured diffraction signals. A first noise weighting function for the first optical metrology tool is generated based on measured diffraction signals measured using the first optical metrology tool. A first measured diffraction signal measured using the first optical metrology tool is obtained. A first adjusted diffraction signal is generated by adjusting the first measured diffraction signal using the first spectra-shift offset and the first noise weighting function.

BACKGROUND

1. Field

The present application relates to optical metrology, and, moreparticularly, to matching optical metrology tools using spectraenhancement.

2. Related Art

Optical metrology involves directing an incident beam at a structure,measuring the resulting diffracted beam, and analyzing the diffractedbeam to determine a feature of the structure. In semiconductormanufacturing, optical metrology is typically used for qualityassurance. For example, after fabricating a structure on asemi-conductor wafer an optical metrology tool is used to determine theprofile of the structure. By determining the profile of the structure,the quality of the fabrication process utilized to form the structurecan be evaluated.

As a result of the broad adoption of optical metrology, one fabricationfacility or site where microelectronics are manufactured typically hasmultiple optical metrology tools in a fleet whose results are usedsomewhat interchangeably. In these cases, it is desirable that theinstruments in the fleet match one another. In the ideal case, if theinstruments were identical, their measurements would match to someuncertainty determined by measurement noise. However, optical metrologytools show deterministic differences, where the difference between themeasurements is greater than the uncertainties of the measurement. Oneapproach to improve matching is to carefully calibrate the tools, sothat the optical characteristics measured by tools are as similar aspossible, even if the details of each of the tools construction dictatethat the detected intensities on the same sample are different. In somesense, this is the goal of calibration.

Calibration is typically done with calibration structures, with theintention that the calibration will remain valid for measurements onvarious application structures. Often calibration structures are one ormore thicknesses of an oxide on a silicon substrate. Applicationstructures can be very different than these simple calibrationstructures. In a fabrication facility, one application structure can beresist on top of a stack of layers for the formation of transistor gatesafter it has been exposed and developed, in order to examine the effectsof adjusting, e.g., focus and dose on the exposure tool. Anotherapplication structure can be shallow isolation trenches in a siliconsubstrate. In general, the optical characteristics of these applicationstructures can be substantially different from one another, and from theoptical characteristics employed in calibration, which is ideallyintended to be valid for all structures.

However, even after the optical metrology tools in a fleet have beencalibrated, their optical characteristics, and subsequently their fitparameters, can differ. Such differences can be an issue for the controlof processes in the fabrication facility. Accordingly, it is desirableto compensate for variations in the optical characteristics of opticalmetrology tools for a given application.

SUMMARY

In one exemplary embodiment, optical metrology tools are matched byobtaining a first set of measured diffraction signals, which wasmeasured using a first optical metrology tool, and a second set ofmeasured diffraction signals, which was measured using a second opticalmetrology tool. A first spectra-shift offset is generated based on thedifference between the first set of measured diffraction signals and thesecond set of measured diffraction signals. A first noise weightingfunction for the first optical metrology tool is generated based onmeasured diffraction signals measured using the first optical metrologytool. A first measured diffraction signal measured using the firstoptical metrology tool is obtained. A first adjusted diffraction signalis generated by adjusting the first measured diffraction signal usingthe first spectra-shift offset and the first noise weighting function.

DESCRIPTION OF DRAWING FIGURES

The present application can be best understood by reference to thefollowing description taken in conjunction with the accompanying drawingfigures, in which like parts may be referred to by like numerals:

FIG. 1 depicts an exemplary optical metrology system;

FIG. 2 depicts an exemplary process of matching optical metrology tools;

FIG. 3 depicts an exemplary fleet of optical metrology tools;

FIG. 4 depict exemplary graphs of spectra-shift offset, noise weightingfunction, and default noise function;

FIG. 5 depicts an exemplary noise profile;

FIG. 6 depicts exemplary noise weighting functions;

FIG. 7 depicts another exemplary noise profile;

FIG. 8 depicts an exemplary process of generating a shift function;

FIG. 9 depicts an exemplary calibration structure mounted on an opticalmetrology tool;

FIG. 10 depicts exemplary calibration structures mounted on an opticalmetrology tool with an r-theta stage;

FIG. 11 depicts exemplary calibration structures mounted on an opticalmetrology tool with an x-y stage; and

FIG. 12 depicts an exemplary calibration wafer.

DETAILED DESCRIPTION

The following description sets forth numerous specific configurations,parameters, and the like. It should be recognized, however, that suchdescription is not intended as a limitation on the scope of the presentinvention, but is instead provided as a description of exemplaryembodiments.

1. Optical Metrology Tools

With reference to FIG. 1, an optical metrology system 100 can be used toexamine and analyze a structure formed on a semiconductor wafer 104. Forexample, optical metrology system 100 can be used to determine one ormore features of a periodic grating 102 formed on wafer 104. Asdescribed earlier, periodic grating 102 can be formed in a test pad onwafer 104, such as adjacent to a die formed on wafer 104. Periodicgrating 102 can be formed in a scribe line and/or an area of the diethat does not interfere with the operation of the die.

As depicted in FIG. 1, optical metrology system 100 can include aphotometric device with a source 106 and a detector 112. Periodicgrating 102 is illuminated by an incident beam 108 from source 106. Theincident beam 108 is directed onto periodic grating 102 at an angle ofincidence θ_(i) with respect to normal {right arrow over (n)} ofperiodic grating 102 and an azimuth angle Φ (i.e., the angle between theplane of incidence beam 108 and the direction of the periodicity ofperiodic grating 102). Diffracted beam 110 leaves at an angle of θ_(d)with respect to normal and is received by detector 112. Detector 112converts the diffracted beam 110 into a measured diffraction signal,which can include reflectance, tan (Ψ), cos(Δ), Fourier coefficients,and the like. Although a zero-order diffraction signal is depicted inFIG. 1, it should be recognized that non-zero orders can also be used.For example, see Ausschnitt, Christopher P., “A New Approach to PatternMetrology,” Proc. SPIE 5375-7, Feb. 23, 2004, pp 1-15, which isincorporated herein by reference in its entirety.

Optical metrology system 100 also includes a processing module 114configured to receive the measured diffraction signal and analyze themeasured diffraction signal. The processing module is configured todetermine one or more features of the periodic grating using any numberof methods which provide a best matching diffraction signal to themeasured diffraction signal. These methods can include a library-basedprocess, or a regression based process using simulated diffractionsignals obtained by rigorous coupled wave analysis and machine learningsystems. See, U.S. Pat. No. 6,943,900, titled GENERATION OF A LIBRARY OFPERIODIC GRATING DIFFRACTION SIGNALS, filed on Jul. 16, 2001, issuedSep. 13, 2005, which is incorporated herein by reference in itsentirety; U.S. Pat. No. 6,785,638, titled METHOD AND SYSTEM OF DYNAMICLEARNING THROUGH A REGRESSION-BASED LIBRARY GENERATION PROCESS, filed onAug. 6, 2001, issued Aug. 31, 2004, which is incorporated herein byreference in its entirety; U.S. Pat. No. 6,891,626, titled CACHING OFINTRA-LAYER CALCULATIONS FOR RAPID RIGROUS COUPLED-WAVE ANALYSES, filedon Jan. 25, 2001, issued May 10, 2005, which is incorporated herein byreference in its entirety; and U.S. patent application Ser. No.10/608,300, titled OPTICAL METROLOGY OF STRUCTURES FORMED ONSEMICONDUCTOR WAFERS USING MACHINE LEARNING SYSTEMS, filed on Jun. 27,2003, which is incorporated herein by reference in its entirety.

2. Matching Optical Metrology Tools

As described above, optical metrology tools in a fleet can becalibrated. However, even after calibration, variations in opticalcharacteristics of the optical metrology tools in the fleet can producevariations in the results obtained using the optical metrology tools.Thus, in one exemplary embodiment, the optical metrology tools in thefleet are matched using spectra enhancement.

In particular, with reference to FIG. 2, an exemplary process 200 isdepicted of matching optical metrology tools in a fleet of opticalmetrology tools using spectra enhancement. In step 202, a first set ofmeasured diffraction signals is obtained. The first set of measureddiffraction signals is measured using a first optical metrology tool inthe fleet. In step 204, a second set of measured diffraction signals isobtained. The second set of measured diffraction signals is measuredusing a second optical metrology tool in the fleet.

For example, FIG. 3 depicts an exemplary fleet 300 having a firstoptical metrology tool 302 and a second optical metrology tool 304. Thefirst and second optical metrology tools 302, 304 can be reflectometers,ellipsometers, and the like. A first set of measured diffraction signalsis measured using first optical metrology tool 302. A second set ofmeasured diffraction signals is measured using second optical metrologytool 304. As also depicted in FIG. 3, processing module 114 can obtainthe first and second sets of measured diffraction signals measured usingfirst optical metrology tool 302 and second optical metrology tool 304,respectively.

It should be recognized that fleet 300 can include any number of opticalmetrology tools, and any number of sets of measured diffraction signalscan be obtained from any number of optical metrology tools. As anexample, fleet 300 is depicted having a third optical metrology tool306, which can be used to measure a third set of measured diffractionsignals.

With reference to FIG. 2, in step 206, a spectra-shift offset isgenerated based on the difference between the first set of measureddiffraction signals and the second set of measured diffraction signals.In the present exemplary embodiment, the first set of measureddiffraction signals was measured from a set of sites on a wafer, and thesecond set of measured diffraction signals was measured from the sameset of sites on the same wafer as the first set of measured diffractionsignals. For example, with reference again to FIG. 3, a set of sites ona wafer can be measured using first optical metrology tool 302, then thesame set of sites on the same wafer can be measured using second opticalmetrology tool 304. In the present exemplary embodiment, thespectra-shift offset is calculated as the average of the differencesbetween each measured diffraction signal in the first set of measureddiffraction signal and each measured diffraction signal in the secondset of measured diffraction signals measured from the same site on thesame wafer.

The spectra-shift offset can be a vector, a table, or graph. Forexample, FIG. 4 depicts an exemplary spectra-shift offset depicted as agraph 402. As depicted in FIG. 4, graph 402 provides the average of thespectra differences between the first and second sets of measureddiffraction signals over the wavelengths used in obtaining the first andsecond sets of measured diffraction signals. It should be recognizedthat any range of wavelengths can be used. See, U.S. Pat. No. 6,792,328,titled METROLOGY DIFFRACTION SIGNAL ADAPTATION FOR TOOL-TO-TOOLMATCHING, filed on Mar. 29, 2002, and issued on Sep. 14, 2004, which isincorporated herein by reference in its entirety.

With reference again to FIG. 3, as mentioned above, fleet 300 caninclude any number of optical metrology tools. In the present exemplaryembodiment, the spectra-shift offset is determined between any one ofthe optical metrology tools in fleet 300 and a reference opticalmetrology tool, which can be one of the optical metrology tools in fleet300 or a separate optical metrology tool. For the sake of the presentexample, assume second optical metrology tool 304 is the referenceoptical metrology tool. Thus, the spectra-shift offset for third opticalmetrology tool 306 is generated based on the difference between thesecond set of measured diffraction signals measured using second opticalmetrology tool 304 and the third set of measured diffraction signalsmeasured using third optical metrology tool 306.

With reference again to FIG. 2, in step 208, a noise weighting functionis generated for the first optical metrology tool based on measureddiffraction signals measured using the first optical metrology tool. Inparticular, the noise weighting function is defined based on the noisethat exists in obtaining the measured diffraction signal using the firstoptical metrology tool. The noise can be related to the hardware used toobtain the measured diffraction signal, such as the optics andelectronics used in the first optical metrology tool. The noise can alsobe related to the feature being measured, such as the phenomenon ofresist bleaching resulting from the source.

With reference to FIG. 5, in the present exemplary embodiment, to definea noise weighting function, a noise profile 502 is first generated. Inparticular, a set of measured diffraction signals are obtained. The setof measured diffraction signals can be obtained in advance from a singlesite on a wafer using the first optical metrology tool. Note, the sitefrom which the set of measured diffraction signal are obtained can be ona different wafer than the wafer on which the feature to be examined isformed.

An average measured diffraction signal is calculated from the set ofmeasured diffraction signals. Noise profile 502 is the differencebetween each of the measured diffraction signals and the averagemeasured diffraction signal. Noise profile 502 depicted in FIG. 5 wascalculated from 50 measured diffraction signals. It should berecognized, however, that any number of measured diffraction signals canbe obtained to generate a noise profile. Because noise profile 502 isgenerated from measured diffraction signals, noise profile 502 takesinto account noise resulting from both hardware related noise andfeature related noise.

After noise profile 502 is obtained, a noise envelope 504 is definedbased on noise profile 502. In the present exemplary embodiment, noiseenvelope 504 is defined using maximum values of noise profile 502 and acurve smoothing technique. It should be recognized, however, that noiseenvelope 504 can be defined using various numerical techniques.

With reference to FIG. 6, in the present exemplary embodiment, a noiseweighting function wb is defined by inverting noise envelope 504 (FIG.5). Noise weighting function wb can be modified to generate additionalweighting functions. For example, noise weighting function wc isgenerated by scaling and truncating noise weighting function wb.

FIG. 7 depicts a noise profile 702 generated by enhancing noise profile502 (FIG. 5) using noise weighting function wc (FIG. 6). In particular,noise profile 502 (FIG. 5) is multiplied by noise weighting function wc(FIG. 6) to generate noise profile 702. As depicted in FIG. 7, noiseweighting function wc reduces the amount of noise and increases theuniformity of noise profile 702. Note, however, that noise weightingfunction wc does not entirely eliminate noise. Completely eliminatingnoise can degrade the measured diffraction signal, which can reduceaccuracy of the optical metrology process.

Thus, in the present exemplary embodiment, the noise weighting functionis modified to remove the desired amount of noise from the measureddiffraction signal without overly degrading the measured diffractionsignal. Additionally, the noise weighting function can be modified toshape the amount of noise reduction of a measured diffraction signal.For example, the noise weighting function can be modified to reduce lessnoise at one portion of a measured diffraction signal compared toanother portion of the measured diffraction signal.

With reference again to FIG. 4, an exemplary noise weighting function404 is depicted as a graph. FIG. 4 also depicts an exemplary defaultnoise function 406, which can be determined empirically, simulated, orderived based upon experience. Noise weighting function 404 and defaultnoise function 406 are depicted over the range of wavelengths used inobtaining the first and second sets of measured diffraction signal. Itshould be recognized that any range of wavelengths can be used. See,U.S. patent application Ser. No. 11/371,752, titled WEIGHTING FUNCTIONTO ENHANCE MEASURED DIFFRACTION SIGNALS IN OPTICAL METROLOGY, filed onMar. 8, 2006, which is incorporated herein by reference in its entirety.

With reference again to FIG. 2, in step 210, a measured diffractionsignal is obtained. The measured diffraction signal is measured usingthe first optical metrology tool. In step 212, an adjusted diffractionsignal is generated by adjusting the measured diffraction signalmeasured using the first optical metrology tool using the firstspectra-shift offset and the noise weighting function. It should berecognized that the first spectra-shift offset and the noise weightingfunction can be applied in any order to the measured diffraction signalto generate the adjusted diffraction signal.

With reference again to FIG. 3, as mentioned above, fleet 300 caninclude any number of optical metrology tools. In the present exemplaryembodiment, the noise weighting function is generated and the adjusteddiffraction signal is generated for any one of the optical metrologytools in fleet 300. For the sake of the present example, a noiseweighting function is generated for third metrology tool 306 in the samemanner as the noise weighting function was generated for secondmetrology tool 304. As described above, a spectra-shift offset for thirdoptical metrology tool 306 is generated based on the difference betweenthe second set of measured diffraction signals measured using secondoptical metrology tool 304 and the third set of measured diffractionsignals measured using third optical metrology tool 306. A measureddiffraction signal is measured using third metrology tool 306, then anadjusted diffraction signal is generated by adjusting the measureddiffraction signal measured using the third optical metrology tool usingthe spectra-shift offset and the noise weighting function for the thirdoptical metrology tool.

3. Generating a Drift Function

Even after calibration, the measurements obtained using an opticalmetrology tool may drift over time. Thus, in one exemplary embodiment, adrift function is generated and used to compensate for drift.

In particular, with reference to FIG. 8, an exemplary process 800 isdepicted of generating a drift function used to compensate for drift inan optical metrology tool. In step 802, a first measured diffractionsignal is obtained of a calibration structure mounted on the firstoptical metrology tool. In step 804, a second measured diffractionsignal is obtained of the calibration structure mounted on the firstoptical metrology tool. The first and second measured diffractionsignals were measured of the calibration structure using the firstoptical metrology tool. The second measured diffraction signal ismeasured later in time than the first measured diffraction signal. Instep 806, a drift function is generated based on the difference betweenthe second measured diffraction signal and the first measureddiffraction signal.

In one exemplary embodiment, after generating the drift function, ameasured diffraction signal measured using the first optical metrologytool is adjusted using the drift function. Thus, in this manner, thedrift in the first optical metrology tool is compensated using the driftfunction. In another exemplary embodiment, the drift function is appliedalong with the spectra-shift and the noise weighting function togenerate the adjusted diffraction signal mentioned above.

With reference now to FIG. 9, an exemplary calibration structure 902 isdepicted. As described above, in one exemplary embodiment, calibrationstructure 902 is mounted on an optical metrology tool. In the exemplaryembodiment depicted in FIG. 9, calibration structure 902 is depictedmounted on a wafer stage 908 of an optical metrology tool. Calibrationstructure 902 is mounted on a support 906 such that the surface ofcalibration structure is substantially in the same plane as the surfaceof wafer 104. As also depicted in FIG. 9, wafer 104 is held by waferchuck 904. The optical metrology device also includes a source 106 anddetector 112. As described above, source 106 directs an incident beam ata structure to be examined, and detector 112 receives the diffractedbeam. In the present exemplary embodiment, source 106 and detector 112are used to measure calibration structure 902 over periods of time togenerate the drift function as described above.

In one exemplary embodiment, multiple calibration structures 902 can bemounted on an optical metrology tool. Each calibration structure 902 canbe used for different applications. With reference to FIG. 10, in anoptical metrology device with an R-theta stage 908, which moves wafer104 relative to source 106 and/or detector 112 in an r (radial)direction and an angle theta direction, as indicated in FIG. 10, themultiple calibration structures 902 can be mounted in-line along aradial direction on support 906. With reference to FIG. 11, in anoptical metrology device with an X-Y stage 908, which moves wafer 104relative to source 106 and/or detector 112 in an x direction and ydirection, as indicated in FIG. 11, the multiple calibration structures902 can be distributed radially around where wafer 104 is held in waferchuck 904 (FIG. 9).

With reference to FIG. 10, multiple calibration structures 902 can bemounted in support 906, such as a bracket. Each calibration structure902 can be a chip with one or more grating structures. Multiple chipscan be formed on a single wafer, diced and used as calibrationstructures 902 on one or more optical metrology tools.

With reference to FIG. 12, in another exemplary embodiment, acalibration wafer 1202 is used to generate a drift function rather thancalibration structures. Similar to the process described for calibrationstructures, calibration wafer 1202 is measured periodically, and a driftfunction is generated based on the difference between two measureddiffraction signals measured from the same site on calibration wafer1202 at two different times.

Calibration wafer 1202 may have several calibration structures formed inselected sites in several sections or sectors. These calibrationstructures may be measured and the statistical average of thedifferences between two measured diffraction signals may be used ingenerating the drift function. For integrated metrology, where ametrology tool is integrated with one or more fabrication tools, one ormore calibration wafers 1202 may be included in the lot of wafersmeasured in a periodic basis in the metrology tool of a fabricationcluster. With a history of how the drift function has changed over timeor a function of some other variable, such as the number of wafer lotsprocessed, a predictive model of the drift function may be developed.The predictive model may be used instead of physically measuring thecalibration structures or calibration wafer 1202.

Although exemplary embodiments have been described, variousmodifications can be made without departing from the spirit and/or scopeof the present invention. Therefore, the present invention should not beconstrued as being limited to the specific forms shown in the drawingsand described above.

1. A method of matching optical metrology tools, the method comprising:obtaining a first set of measured diffraction signals, wherein the firstset of measured diffraction signals was measured using a first opticalmetrology tool; obtaining a second set of measured diffraction signals,wherein the second set of measured diffraction signals was measuredusing a second optical metrology tool; generating a first spectra-shiftoffset based on the difference between the first set of measureddiffraction signals and the second set of measured diffraction signals;generating a first noise weighting function for the first opticalmetrology tool based on measured diffraction signals measured using thefirst optical metrology tool; obtaining a first measured diffractionsignal measured using the first optical metrology tool; and generating afirst adjusted diffraction signal by adjusting the first measureddiffraction signal using the first spectra-shift offset and the firstnoise weighting function.
 2. The method of claim 1, further comprising:obtaining a third set of measured diffraction signals, wherein the thirdset of measured diffraction signals was measured using a third opticalmetrology tool; generating a second spectra-shift offset based on thedifference between the third set of measured diffraction signals and thesecond set of measured diffraction signals; generating a second noiseweighting function for the third optical metrology tool based onmeasured diffraction signals measured using the third optical metrologytool; obtaining a second measured diffraction signal measured using thethird optical metrology tool; and generating a second adjusteddiffraction signal by adjusting the second measured diffraction signalusing the second spectra-shift offset and the second noise weightingfunction.
 3. The method of claim 1, wherein the first spectra-shiftoffset is a vector, table, or graph.
 4. The method of claim 1, whereinthe first set of measured diffraction signals was measured from a set ofsites on a wafer, wherein the second set of measured diffraction signalswas measured from the same set of sites on the same wafer as the firstset of measured diffraction signals.
 5. The method of claim 4, whereingenerating a first spectra-shift offset comprises: calculatingdifferences between each measured diffraction signal in the first set ofmeasured diffraction signals and each measured diffraction signal in thesecond set of measured diffraction signals measured from the same siteon the same wafer.
 6. The method of claim 1, wherein generating a firstnoise weighting function comprises: generating a noise profile based onthe measured diffraction signals measured using the first opticalmetrology tool; defining a noise envelope based on the noise profile;and defining the first noise weighting function based on the noiseenvelope.
 7. The method of claim 6, wherein the measured diffractionsignals measured using the first optical metrology tool are the same ora subset of the first set of measured diffraction signals.
 8. The methodof claim 6, wherein the measured diffraction signals measured using thefirst optical metrology tool are different than the first set ofmeasured diffraction signals.
 9. The method of claim 1, furthercomprising: obtaining a measurement of a calibration structure mountedon the first optical metrology tool using the first optical metrologytool; generating a drift function based on the obtained measurement ofthe calibration structure; and generating the first adjusted diffractionsignal by adjusting the first measured diffraction signal using thedrift function.
 10. The method of claim 9, wherein generating the driftfunction comprises: comparing the obtained measurement of thecalibration structure to a previous measurement of the calibrationstructure measured using the first optical metrology tool.
 11. Themethod of claim 9, wherein the calibration structure is a chip mountedon a wafer stage.
 12. The method of claim 11, wherein the chip includesa grating structure.
 13. The method of claim 9, wherein the calibrationstructure includes a plurality of calibration structures, wherein eachcalibration structure is different, and the different calibrationstructures are measured for different applications.
 14. Acomputer-readable storage medium having computer executable instructionsfor matching optical metrology tools, comprising instructions for:obtaining a first set of measured diffraction signals, wherein the firstset of measured diffraction signals was measured using a first opticalmetrology tool; obtaining a second set of measured diffraction signals,wherein the second set of measured diffraction signals was measuredusing a second optical metrology tool; generating a first spectra-shiftoffset based on the difference between the first set of measureddiffraction signals and the second set of measured diffraction signals;generating a first noise weighting function for the first opticalmetrology tool based on measured diffraction signals measured using thefirst optical metrology tool; obtaining a first measured diffractionsignal measured using the first optical metrology tool; and generating afirst adjusted diffraction signal by adjusting the first measureddiffraction signal using the first spectra-shift offset and the firstnoise weighting function.
 15. The computer-readable storage medium ofclaim 14, further comprising instructions for: obtaining a third set ofmeasured diffraction signals, wherein the third set of measureddiffraction signals was measured using a third optical metrology tool;generating a second spectra-shift offset based on the difference betweenthe third set of measured diffraction signals and the second set ofmeasured diffraction signals; generating a second noise weightingfunction for the third optical metrology tool based on measureddiffraction signals measured using the third optical metrology tool;obtaining a second measured diffraction signal measured using the thirdoptical metrology tool; and generating a second adjusted diffractionsignal by adjusting the second measured diffraction signal using thesecond spectra-shift offset and the second noise weighting function. 16.The computer-readable storage medium of claim 14, further comprisinginstructions for: obtaining a measurement of a calibration structuremounted on the first optical metrology tool using the first opticalmetrology tool; generating a drift function based on the obtainedmeasurement of the calibration structure; and generating the firstadjusted diffraction signal by adjusting the first measured diffractionsignal using the drift function.
 17. A system of matching opticalmetrology tool, the system comprising: a computer-readable storagemedium configured to store: a first spectra-shift offset, wherein thefirst spectra-shift offset was generated based on the difference betweena first set of measured diffraction signals measured using a firstoptical metrology tool and a second set of measured diffraction signalsmeasured using a second optical metrology tool; and a first noiseweighting function for the first optical metrology tool, wherein thefirst noise weighting function was generated based on measureddiffraction signals measured using the first optical metrology tool; anda processor configured to: obtain a first measured diffraction signalmeasured using the first optical metrology tool; and generate a firstadjusted diffraction signal by adjusting the first measured diffractionsignal using the first spectra-shift offset and the first noiseweighting function.
 18. The system of claim 17, wherein thecomputer-readable storage medium is further configured to store: asecond spectra-shift offset, wherein the second spectra-shift offset wasgenerated based on the difference between a third set of measureddiffraction signals measured using a third optical metrology tool andthe second set of measured diffraction signals measured using the secondoptical metrology tool; and a second noise weighting function for thethird optical metrology tool, wherein the second noise weightingfunction was generated based on measured diffraction signals measuredusing the third optical metrology tool; and wherein the processor isfurther configured to: obtain a second measured diffraction signalmeasured using the third optical metrology tool; and generate a secondadjusted diffraction signal by adjusting the second measured diffractionsignal using the second spectra-shift offset and the second noiseweighting function.
 19. The system of claim 17, wherein thecomputer-readable storage medium is further configured to store a driftfunction for the first optical metrology tool, wherein the driftfunction was generated based on obtained measurements of a calibrationstructure mounted on the first optical metrology tool using the firstoptical metrology tool at different times, and wherein the processor isfurther configured to generate the first adjusted diffraction signal byadjusting the first measured diffraction signal using the driftfunction.
 20. A method of matching optical metrology tools, the methodcomprising: obtaining a first set of measured diffraction signals,wherein the first set of measured diffraction signals was measured usinga first optical metrology tool; obtaining a second set of measureddiffraction signals, wherein the second set of measured diffractionsignals was measured using a second optical metrology tool; generating afirst spectra-shift offset based on the difference between the first setof measured diffraction signals and the second set of measureddiffraction signals; generating a first noise weighting function for thefirst optical metrology tool based on measured diffraction signalsmeasured using the first optical metrology tool; and storing the firstspectra-shift offset and the first noise weighting function in acomputer-readable storage medium.
 21. The method of claim 20, furthercomprising: obtaining a first measured diffraction signal measured usingthe first optical metrology tool; retrieving the first spectra-shiftoffset and the first noise weighting function from the computer-readablestorage medium; and generating a first adjusted diffraction signal byadjusting the first measured diffraction signal using the retrievedfirst spectra-shift offset and the first noise weighting function. 22.The method of claim 20, further comprising: obtaining a third set ofmeasured diffraction signals, wherein the third set of measureddiffraction signals was measured using a third optical metrology tool;generating a second spectra-shift offset based on the difference betweenthe third set of measured diffraction signals and the second set ofmeasured diffraction signals; generating a second noise weightingfunction for the third optical metrology tool based on measureddiffraction signals measured using the third optical metrology tool; andstoring the second spectra-shift offset and the second noise weightingfunction in the computer-readable storage medium.
 23. The method ofclaim 22, further comprising: obtaining a second measured diffractionsignal measured using the third optical metrology tool; retrieving thesecond spectra-shift offset and the second noise weighting function fromthe computer-readable storage medium; and generating a second adjusteddiffraction signal by adjusting the second measured diffraction signalusing the retrieved second spectra-shift offset and the second noiseweighting function.
 24. The method of claim 20, further comprising:obtaining a measurement of a calibration structure mounted on the firstoptical metrology tool using the first optical metrology tool;generating a drift function based on the obtained measurement of thecalibration structure; and storing the drift function in thecomputer-readable storage medium.
 25. The method of claim 24, furthercomprising: retrieving the drift function; and generating the firstadjusted diffraction signal by adjusting the first measured diffractionsignal using the retrieved drift function.
 26. A computer-readablestorage medium having computer executable instructions for matchingoptical metrology tools, comprising instructions for: obtaining a firstset of measured diffraction signals, wherein the first set of measureddiffraction signals was measured using a first optical metrology tool;obtaining a second set of measured diffraction signals, wherein thesecond set of measured diffraction signals was measured using a secondoptical metrology tool; generating a first spectra-shift offset based onthe difference between the first set of measured diffraction signals andthe second set of measured diffraction signals; generating a first noiseweighting function for the first optical metrology tool based onmeasured diffraction signals measured using the first optical metrologytool; and storing the first spectra-shift offset and the first noiseweighting function in a computer-readable storage medium.
 27. Thecomputer-readable storage medium of claim 26, further comprisinginstructions for: obtaining a first measured diffraction signal measuredusing the first optical metrology tool; retrieving the firstspectra-shift offset and the first noise weighting function from thecomputer-readable storage medium; and generating a first adjusteddiffraction signal by adjusting the first measured diffraction signalusing the retrieved first spectra-shift offset and the first noiseweighting function.
 28. The computer-readable storage medium of claim26, further comprising instructions for: obtaining a third set ofmeasured diffraction signals, wherein the third set of measureddiffraction signals was measured using a third optical metrology tool;generating a second spectra-shift offset based on the difference betweenthe third set of measured diffraction signals and the second set ofmeasured diffraction signals; generating a second noise weightingfunction for the third optical metrology tool based on measureddiffraction signals measured using the third optical metrology tool; andstoring the second spectra-shift offset and the second noise weightingfunction in the computer-readable storage medium.
 29. Thecomputer-readable storage medium of claim 28, further comprisinginstructions for: obtaining a second measured diffraction signalmeasured using the third optical metrology tool; retrieving the secondspectra-shift offset and the second noise weighting function from thecomputer-readable storage medium; and generating a second adjusteddiffraction signal by adjusting the second measured diffraction signalusing the retrieved second spectra-shift offset and the second noiseweighting function.
 30. The computer-readable storage medium of claim26, further comprising instructions for: obtaining a measurement of acalibration structure mounted on the first optical metrology tool usingthe first optical metrology tool; generating a drift function based onthe obtained measurement of the calibration structure; and storing thedrift function in the computer-readable storage medium.
 31. Thecomputer-readable storage medium of claim 30, further comprisinginstructions for: retrieving the drift function; and generating thefirst adjusted diffraction signal by adjusting the first measureddiffraction signal using the retrieved drift function.
 32. A system formatching optical metrology tools, the system comprising: a first opticalmetrology tool configured to measure a first set of measured diffractionsignals; a second optical metrology tool configured to measure a secondset of measured diffraction signals; a processor module configured to:generate a first spectra-shift offset based on the difference betweenthe first set of measured diffraction signals and the second set ofmeasured diffraction signals; generate a first noise weighting functionfor the first optical metrology tool based on measured diffractionsignals measured using the first optical metrology tool; and store thefirst spectra-shift offset and the first noise weighting function in acomputer-readable storage medium.
 33. A method of matching opticalmetrology tools, the method comprising: obtaining a first measureddiffraction signal, wherein the first measured diffraction signal wasmeasured using a first optical metrology tool; obtaining a firstspectra-shift offset for the first optical metrology tool, wherein thefirst spectra-shift offset was generated based on the difference betweena first set of measured diffraction signals measured using the firstoptical metrology tool and a second set of measured diffraction signalsmeasured using a second optical metrology tool; obtaining a first noiseweighting function for the first optical metrology tool, wherein thefirst noise weighting function was generated based on measureddiffraction signals measured using the first optical metrology tool; andgenerating a first adjusted diffraction signal by adjusting the firstmeasured diffraction signal using the first spectra-shift offset and thefirst noise weighting function.
 34. The method of claim 33, furthercomprising: obtaining a second measured diffraction signal, wherein thesecond measured diffraction signal was measured using a third opticalmetrology tool; obtaining a second spectra-shift offset for the thirdoptical metrology tool, wherein the second spectra-shift offset wasgenerated based on the difference between a third set of measureddiffraction signals measured using the third optical metrology tool andthe second set of measured diffraction signals measured using the secondoptical metrology tool; obtaining a second noise weighting function forthe third optical metrology tool based on measured diffraction signalsmeasured using the third optical metrology tool; and generating a secondadjusted diffraction signal by adjusting the second measured diffractionsignal using the second spectra-shift offset and the second noiseweighting function.
 35. A computer-readable storage medium havingcomputer executable instructions for matching optical metrology tools,comprising instructions for: obtaining a first measured diffractionsignal, wherein the first measured diffraction signal was measured usinga first optical metrology tool; obtaining a first spectra-shift offsetfor the first optical metrology tool, wherein the first spectra-shiftoffset was generated based on the difference between a first set ofmeasured diffraction signals measured using the first optical metrologytool and a second set of measured diffraction signals measured using asecond optical metrology tool; obtaining a first noise weightingfunction for the first optical metrology tool, wherein the first noiseweighting function was generated based on measured diffraction signalsmeasured using the first optical metrology tool; and generating a firstadjusted diffraction signal by adjusting the first measured diffractionsignal using the first spectra-shift offset and the first noiseweighting function.
 36. The computer-readable storage medium of claim35, further comprising instructions for: obtaining a second measureddiffraction signal, wherein the second measured diffraction signal wasmeasured using a third optical metrology tool; obtaining a secondspectra-shift offset for the third optical metrology tool, wherein thesecond spectra-shift offset was generated based on the differencebetween a third set of measured diffraction signals measured using thethird optical metrology tool and the second set of measured diffractionsignals measured using the second optical metrology tool; obtaining asecond noise weighting function for the third optical metrology toolbased on measured diffraction signals measured using the third opticalmetrology tool; and generating a second adjusted diffraction signal byadjusting the second measured diffraction signal using the secondspectra-shift offset and the second noise weighting function.
 37. Asystem for matching optical metrology tools, the system comprising: afirst optical metrology tool configured to measure a first measureddiffraction signal; and a processor module configured to: obtain a firstspectra-shift offset for the first optical metrology tool, wherein thefirst spectra-shift offset was generated based on the difference betweena first set of measured diffraction signals measured using the firstoptical metrology tool and a second set of measured diffraction signalsmeasured using a second optical metrology tool; obtain a first noiseweighting function for the first optical metrology tool, wherein thefirst noise weighting function was generated based on measureddiffraction signals measured using the first optical metrology tool; andgenerate a first adjusted diffraction signal by adjusting the firstmeasured diffraction signal using the first spectra-shift offset and thefirst noise weighting function.